(a) Field of the Invention
The present invention relates to a receiving system for estimating symbol timing in a feed-forward manner and a method for estimating the symbol timing in a feed-forward manner. More specifically, the present invention relates to a receiving system for estimating symbol timing in a feed-forward manner and simply estimating the symbol timing, and a method for estimating the symbol timing in a digital phase modulation communication system.
(b) Description of the Related Art
In general, a receiving system in a communication system receives signals, converts them into baseband signals, performs symbol timing estimation, interpolation, and demodulation in the baseband, and extracts available target signals.
Since symbol intervals of a communication system become very short as high-speed information transmission develops, it is required to accurately control the symbol timing for extracting desired signals so as to extract accurate data, and algorithms for estimating symbol timing are also required.
The algorithms are classified as a feedback configuration and a feed-forward configuration, and the symbol timing estimation algorithm of the feed-forward configuration can operate with two samples per symbol, but the generally-used symbol timing estimation algorithms of the feedback configuration require at least three samples per symbol in most cases, wherein the symbol represents data of a plurality of bits.
However, since it is difficult to use the feedback symbol timing estimation algorithm in the packet transmission communication system, a feed-forward symbol timing estimation algorithm is used, one example of which is disclosed in a thesis entitled “Digital filter and square timing recovery” from IEEE Trans. Communications, Vol. 36, No. 5, Pp. 605–612, of May 1988.
The thesis relates to a symbol timing recovery algorithm suitable for high-speed data transmission, and by applying the algorithm, a free-running oscillator that is not influenced by symbol timing control operation can be used for an analog-digital (A/D) converter.
In general, in the case of using a feedback symbol timing recovery method, sampling time of the A/D converter is controlled by symbol timing errors, and hence a free-running oscillator cannot be used in this case. Using a free-running oscillator for A/D conversion necessitates enabling feed-forward symbol timing recovery. Since a feedback configuration generally requires a long preamble which is used only for signal synchronization but not for user information, it is suitable to use a feed-forward configuration to perform symbol timing recovery in the case of packet transmission.
When the number of required samples per symbol is low, the operation speed of the A/D converter may be slow, and the operation speed of the symbol timing estimation algorithm may also be slow, thereby providing easy implementation.
However, the prior art disclosed in the above thesis requires at least 3 to 4 samples to accurately estimate symbol timing, and accordingly, it problematically generates heavy loads in providing a whole communication service since it has many samples to process for a predetermined time.
That is, the detailed symbol timing estimation algorithm suggested by the prior art thesis is expressed in Equation 1.
                              τ          ^                =                              1                          2              ⁢              π                                ⁢                      arg            ⁡                          (                                                ∑                                      k                    =                    0                                    L                                ⁢                                                                  ⁢                                  |                                      r                    ⁡                                          (                                              kT                        s                                            )                                                        ⁢                                      |                    2                                    ⁢                                      ⅇ                                                                  -                        j2                                            ⁢                                                                                          ⁢                      π                      ⁢                                                                        T                          s                                                T                                                                                                        )                                                          Equation        ⁢                                  ⁢        1                            where r( ) represents a complex number value of a receiving signal, |x| denotes a magnitude of a complex number x, Ts shows a sampling interval, and T indicates a symbol interval.        
      If    ⁢                  ⁢          T      s        =            T      2        ,                  ⁢          ⅇ                        -          jk2                ⁢                                  ⁢        π        ⁢                              T            s                    T                                    always represents a real number, and hence the estimate of the above equation only is either 0 or ½, thereby failing to execute a normal operation.        
Therefore, in order for the equation to normally function, a condition
            T      s        T    <      1    2  must be satisfied, and since a condition
            T      s        T    =      1    4  is generally used, 4 samples are required.